Each of sense amplifiers employed in a semiconductor memory device is of a kind of an amplifying circuit for sensing a microsignal. As typical sense amplifiers, there are known those such as a bit line sense amplifier and an I/O sense amplifier. The bit line sense amplifier is used to sense a microsignal on a bit line pair, which is produced from a memory cell, whereas the I/O sense amplifier is used to sense a signal transmitted over a data bus line pair and amplify it.
As these sense amplifiers, there are known those of current and voltage types. A current type sense amplifier related to this invention has been disclosed in a reference 1 (A7-ns 140-mW 1-Mb CMOS SRAM with Current Sense Amplifier, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 27, NO. 11, NOVEMBER 1992, pp. 1511 through 1518), for example. This type of current sense amplifier is used to amplify the amplitude of current flowing in each bit line depending on a difference between currents which flow in the bit line pair, and is particularly effective when it is activated at a low voltage and its amplitude is small.